Cow Rumen

What would happen if you ate nothing but grass and hay? You would probably starve. But cows eat grass and hay almost exclusively, yet they grow big and fat. Why is this? Because cows, like other ruminant animals, have a special type of stomach called a rumen, which is home to billions of microbes which can eat grass and hay. These bacteria, fungi and protists provide nutrients that the cow can digest. Without these microbes, the cow would die.

Here are just a few microbes living in a rumen:

Rumen symbiontsMany diverse microbes live together in close association in the rumen.

Holy Cow

Over 103 million head of cattle valued at $600 per head roam the United States, supplying Americans with dairy products and meat. With two head of cattle for every five people, the country has a big interest in keeping its cows healthy and fed. Learning about the ecology of the rumen is one way scientists can help keep cows healthy.

Hay Munchers

Rumen microbes help the cow eat hay, which is made of cellulose and other polymers, which are long molecules that the animal cannot digest, but microbes can. The microbes break down the cellulose into smaller bits which the cow can take in, or absorb. The microbes use special proteins called enzymes to break down cellulose into small bits.

Other Ruminants

Cows are not the only ruminant animals. Other ruminants include gazelles, giraffes, moose, antelope, caribou, sheep, goats, and deer. Other herbivores (plant eating animals) such as kangaroos, camels and llamas have stomachs that are similar to rumens.

Why is it called a ruminant?

Because there is "room in it"! Actually, the term rumen is Latin for throat or gullet. The rumen is the first of four stomachs. Ruminant animals have a lot of room in their rumens. The average cow rumen can hold over 160 liters (40 gallons)! The rumen is the first of four parts of the stomach in ruminant animals. Food is partly digested in the rumen and then spit up (regurgitated) for further chewing.

Many Microbial Munchers

The rumen is home to billions and billions of microbes, including bacteria, protists, fungi, and viruses. These many different rumen microbes form a complex community of organisms that interact with one another, helping the animal digest its food.

This graph shows the relative numbers of different microbes in a cow's rumen. The scale on the left is logarithmic, meaning each higher line represents 100 times more organisms than the line beneath it. The total number of humans on Earth is given as a comparison. Note that there are more bacteria in one rumen than there are people on Earth.

Stinky Place

The rumen stinks. This is because microbes in the rumen produce stinky organic acids. The billions of microbes in the rumen quickly use up all the oxygen. Because there is no oxygen, the rumen is anaerobic. When oxygen is lacking, microbes must get their energy from anaerobic respiration or from fermentation. In anaerobic respiration, microbes breathe compounds other than oxygen for energy. Fermentation is the breaking down of organic molecules into smaller molecules such as organic acids like butyric acid and valeric acid that stink.

Burnable Burps

One of the effects of life without oxygen is that methane is produced. Methane is a gas produced by certain microbes called methanogens. Methanogens are members of the Archaea. The Archaea, also known as Archaebacteria, are a group of bacteria which look similar to eubacteria, but are remarkably different at the molecular level. Archaea and bacteria are presumed to have diverged billions of years ago.